I also experienced a mysterious anthelia (waving hand coral) meltdown in my tank earlier this year which occurred seemingly overnight and without known reason.
Anthelia had been thriving and growing very invasive-like in my tank since 2006. One day about 7 or 8 months ago the entire colony tank-wide took a big dive. The only subtle warning I had was that the day prior the tank water was emitting a 'coral' smell (something it's never done before but everything looked fine in the tank so I didn't think anything of it). The next day essentially the entire anthelia colony was disintegrated and dissolving.
There had been no changes in my tank husbandry so I not sure what caused this.
The widespread colony was reduced to a tiny fragment group comprised of 4 or 5 polyps growing on the side of the glass.
Since then this coral has been making a slow comeback and is growing in small patches in various places in the tank.
Interestingly, I also have a huge pulsating xenia colony in the tank. The xenia (despite being a close relative of anthelia) was completely unaffected by this anthelia-meltdown incident.